Improved vibroacoustography imaging for nondestructive inspection of materials

We report a method to improve image quality in the nondestructive investigation and visualization of defects using vibroacoustography (VA). Vibroacoustography is an ultrasound-based imaging technique that uses the dynamic (oscillatory) radiation force of low-frequency excitation (within kilohertz range) to remotely vibrate objects and detect the ensuing acoustic emission. This technique is nondestructive and noncontact and has shown numerous capabilities to produce high-resolution images of different types of materials. However, for reflective materials, ultrasound reflects back and forth between the object and transducer, thus establishing standing waves. This phenomenon produces an artifact in the shape of false contours in the VA images. The goal of this study is to investigate the formation of the standing wave artifacts and develop a process called chirp imaging to improve defect visibility and flaw detection capability. Chirp VA experiments are performed on a flawed fiber-reinforced ceramic composite plate and on an electronic chip. To assess the efficacy of the chirp imaging process in removing the standing-wave artifact, the resulting chirp images are compared to "fixed frequency" VA images. Results show that the false contour can be significantly reduced in the image, thus remarkably improving image quality and flaw detection.